The present invention relates to a phase change type optical recording material, and an optical recording medium using such a recording material.
Highlight is recently focused on optical recording media capable of recording information at a high density and erasing the recorded information for overwriting. One typical rewritable optical recording medium is of the phase change type wherein a laser beam is directed to the recording layer to change its crystallographic state whereupon a change of reflectance by the crystallographic change is detected for reading. Optical recording media of the phase change type are of great interest since they can be overwritten by modulating the intensity of a single light beam and the optical system of the drive unit used for their operation is simple as compared with magneto-optical recording media.
Most optical recording media of the phase change type use Ge-Te base materials which provide a substantial difference in reflectance between crystalline and amorphous states and have a relatively stable amorphous state. In recent years, it has been proposed to use compounds known as chalcopyrites.
Chalcopyrite compounds are extensively investigated as compound semiconductor materials, and applied to solar batteries and the like. The chalcopyrite compounds are composed of Ib-IIIb-VIb.sub.2 or IIb-IVb-Vb.sub.2 as expressed in terms of the Groups of the Periodic Table and have two stacked diamond structures. The structure of chalcopyrite compounds can be readily determined by X-ray structural analysis and their basic characteristics are described in Physics, Vol. 8, No. 8 (1987), page 441, Denki Kagaku (Electrochemistry), Vol. 56, No. 4 (1988), page 228, and other literature.
Among the chalcopyrite compounds, AgInTe.sub.2 is known to be applicable as a recording material by diluting it with antimony or Sb or bismuth or Bi. The resulting optical recording media are generally operated at a linear velocity of about 7 m/s. See JP-A's 3-240590, 3-99884, 3-82593, 3-73384 and 4-151286.
In addition to these phase change type optical recording media using chalcopyrite compounds, JP-A's 4-267192, 4-232779 and 6-166268 disclose phase change type optical recording media wherein an AgSbTe.sub.2 phase forms upon the crystallization of a recording layer.
For the phase change type optical recording medium, a recording layer is usually formed using vacuum deposition equipment or the like, and so is in an amorphous state just after formation. The recording layer must be crystallized by an operation generally known as initialization before the recording media can be utilized as rewritable media. In the initialization operation the recording layer is crystallized by heating, and slow cooling. After initialization, the recording layer is irradiated with rewriting or overwriting laser beam, whereupon the recording layer melts at a spot with recording power applied thereon. Then, the temperature of the region drops quickly so that the region becomes substantially amorphous or microcrystalline to form a recorded mark having a lowered reflectance. On the other hand, no change is found in a region with erasing power applied thereon, so that the reflectance of the region can be on much the same level as obtained upon initialization. When it is desired to rewrite the recorded information, recording power is applied on another region of the recording layer which is to become a new recorded mark while erasing power is applied on the remaining regions. Whether the recording material is crystalline or amorphous (microcrystalline) before irradiation, all regions with recording power applied thereon provide recorded marks of amorphous or microcrystalline nature while all regions with erasing power applied thereon become crystalline, so that overwrite recording can be done.
Among basic characteristics the phase change type optical recording medium should have, there are the degree of modulation, recording sensitivity, etc. Insofar as the change phase type optical recording medium forming an AgSbTe.sub.2 phase is concerned, however, nothing is known about what crystallographic structure of a recording layer makes these basic properties satisfactory; it is still difficult to make sure of an optical recording medium well improved in terms of such properties.
Prior art phase change type recording media require to repeat rewriting several times after initialization until a constant rate of erasure is reached. In most cases, rewriting is generally repeated about ten times before performance rating is carried out. The reason why the rate of erasure of the phase change type optical recording medium with the formation of an AgSbTe.sub.2 phase remains unstable upon rewriting immediately after initialization appears to be that the formation of an AgSbTe.sub.2 or In-Te crystalline phase is incomplete, although details of the mechanism involved have yet to be clarified.
One object of the present invention is to achieve a phase change type optical recording medium having high degree of modulation and recording sensitivity. Another object of the present invention is to achieve a phase change type optical recording medium having already stable write/read characteristics at the time of the first rewriting.